I have a child with a sept B-day who I started on time. He was/is totally ready! The norm here seems to be red shirting, but for my child it was not the right thing. My youngest has a very late b-day 2 weeks from the cut off. I will wait until right before K to decided. I will go by whether she is ready, not the fact that she will be the youngest. Somebody has to be the youngest.
Kindergarten readiness - Page 2
I think a good kindergarten readiness test can help guide parents as they make the red-shirt-or-don't-red-shirt decision. If your child is developmentally ready, I'm not sure why you would hold him back a year. But if he is still lacking important skills, maybe an extra year could really help -- do extra work like kindergarten worksheets, flash cards, socialization, etc so he's ready to thrive the following year.